A wireless communications system may generally comprise a set of “mobile stations,” typically mobile stations are the endpoints of a communication path, and a set of “base stations,” (also known as “repeaters”) typically stationary and the intermediaries by which a communication path to a mobile station (MS) may be established or maintained. One such type of system is a time division multiple access (TDMA) communication system where the radio medium (or RF frequency) is divided into time slots to carry the communications of the system. Because the communication system carries many communications at one time, a mobile station may want to monitor other communications in the system. Scan is a feature that allows a mobile station to monitor other communications in the system.
MSs of the wireless communications system utilize a feature termed “scan” where an MS locks on to a specific RF frequency (also termed a “channel”) and inspects activity that may be present on the channel. The RF frequencies that the MS inspects may be associated with more than one wireless communications system. For example, an MS may inspect RF frequencies associated with the Schaumburg fire department and RF frequencies associated with the Rolling Meadows fire department.
While a mobile station is engaged in a call with a first base station, the mobile station will periodically scan neighboring base stations, e.g. a second base station, for other communications. The mobile station locks onto a specific RF frequency of the second base station and determines whether there is any RF energy on the channel. If there is not, then the mobile station returns to listening to the first base station and continues being engaged in the call. If there is RF energy and that RF energy is above a threshold, then the mobile station continues listening to the channel and determines whether the RF energy is of interest by synchronizing to the channel and decoding a message to determine whether the RF energy is a communication addressed to the mobile station. This process of determining whether the RF energy is a communication address to the mobile station may take a long time which is time away from the call that the mobile station is engaged in with the first base station. Such time away is termed an “audio hole” of the system. If the RF energy is not a communication that is addressed to the mobile station, then the mobile station returns to the call that it was previously engaged in and to listening to the first base station.
In the prior art, the mobile station remembers that the last time the mobile station performed a scan of the second base station and that the RF energy that was associated with the second base station was not of interest to the mobile station. Thus, the next time the mobile station performs scan and listens to the second base station, if RF energy is present, then the mobile station assumes that the RF energy is a continuing communication which is of no interest to the mobile station and quickly returns to the call that it is listening to with the first base station. Such an assumption is made so that the audio hole of the system is minimized.
Assuming that a previous determination of RF energy of no interest for a subsequent inspection of the channel where RF energy is found is not a good assumption for TDMA systems, since the fact that RF energy is available may be attributed to a new call (that may be of interest) in a different time slot as opposed to the same call (that is of no interest) in the same time slot. Thus, to determine whether the RF energy is of a new call or the same call, the mobile station needs to synchronize and decode the RF energy to determine whether the RF energy is a communication that is of interest to the mobile station which causes the function of scan to take more time and causes the “audio hole” problem to be compounded in TDMA systems.
Accordingly, there exists a need for scanning a TDMA channel which improves the amount of time that an MS spends scanning.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate identical elements.